Inducer wheels or impellers have been used with condensing furnaces to draw air and gas through a heat exchanger. In a broader sense, inducer wheels or impellers create a pressure differential in a fluid in order to move the fluid from one location to another.
Inducer wheels usually include a generally disk shaped rotor and a centrally disposed hub by which the inducer wheel is mounted to a rotatable shaft. The rotor section includes a plurality of blades generally radially extending from the hub which function like the blades of a fan.
In some inducer wheels, the rotor portion is fashioned substantially of plastic, and the hub is fashioned of metal. The rotor may be secured to the hub by placing the hub in a mold for forming the rotor from liquid plastic and allowing the plastic to flow around the hub and then-harden. The periphery of the hub usually includes a series of ridges or depressions that form a drive gear whereby rotation of the shaft causes concurrent rotation of the hub, which in turn causes concurrent rotation of the rotor by contact of the rotor with the series of ridges or depressions in the drive gear of the hub.
Typically, the rotatable shaft upon which the inducer wheel is mounted is generally cylindrical in shape, but is provided with a flattened peripheral region extending longitudinally along the shaft. The hub is also provided with a substantially cylindrical cavity in which the shaft is adapted to extend, with very little tolerance between the wall of the cavity and the periphery of the shaft. The hub cavity is also provided with a flattened section that conforms with the flattened section of the shaft so that the hub does not slip relative to the shaft when the shaft is rotated. A set screw may extend radially through the hub to selectively clamp the hub to the shaft at a selected location along the shaft.
Two problems have arisen in connection with the foregoing types of inducer wheel assemblies. First, the hub with a flattened portion of the cavity wall must be made of powdered metal that solidifies when molded. A probe must be disposed in the mold, with the powdered metal being solidified around the probe, whereafter the probe is withdrawn. However, in order for the probe to be withdrawn, the probe necessarily possesses a draft (i.e., a tapered inner diameter). As such, the tolerance between the actual rotatable shaft periphery and the interior walls of the cavity increases along the shaft length, thereby causing a relatively loose fit between the hub and the shaft and inducing wobble or vibration in the inducer wheel when the shaft is rotated. Such vibrations cause the plastic forming the rotor in the vicinity of the drive gear to become cracked and eventually break away, requiring replacement. Second, in the powder metal hub designs, the amount of plastic in the rotor that intimately contacts the hub in the region of the drive gear has been relatively insufficient, which likewise has resulted in cracking and breakage of the plastic in the region of the drive gear.
The present invention relates to an inducer wheel or impeller in which the hub may be formed by machining, rather than by molding, and which possesses a generally cylindrical cavity having a flattened section without a tapered inner diameter. Consequently, the hub may be mounted on a rotatable shaft having a corresponding flattened portion with relatively small tolerance between the periphery of the shaft and the cavity wall of the hub uniformly along the shaft length. A plastic rotor may be molded onto the hub such that the plastic abuttingly contacts the flattened region of the rotatable shaft and surrounds and covers a relatively large peripheral area of the hub. A method of making such an inducer wheel is disclosed, and the inducer wheel made by such a method is also disclosed.